Oxidative Modification of Guanine Bases Initiated by Oxyl Radicals Derived from Photolysis of Azo Compounds

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Abstract
Oxidative damage to guanine bases initiated by photolysis of the water-soluble radical generator 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH) has been investigated by laser kinetic spectroscopy. In the neutral oxygenated aqueous solutions, 355 nm laser flash photolysis of AAPH generates a whole spectrum of free radicals including 2-amidinoprop-2-peroxyl (ROO•), 2-amidinoprop-2-oxyl (RO•), and superoxide (O2•−) radicals. These oxyl radicals with negligible absorption in a near UV−visible range were monitored in the reactions leading to the products with characteristic absorption spectra. This approach reveals that RO• radicals induce fast one-electron oxidation of 2′-deoxyguanosine (dG) to form guanine neutral radicals, dG(−H)•. In contrast, ROO• radicals do not react at observable rates with dG. The O2•− radicals were detected using a classical test reaction with tetranitromethane to form nitroform. The major pathway for formation of the end-products of guanine oxidation is the combination of the G(−H)• and O2•− radicals to form 2,5-diamino-4H-imidazolone (Iz). This mechanism was confirmed by analysis of the end-products produced by oxidation of two substrates: (1) the guanosine derivative 2′,3′,5′-tri-O-acetylguanosine (tri-O-Ac-G) and (2) the 5′-d(CCATCGCTACC) sequence. The major products isolated by HPLC and identified by mass spectrometry methods were the tri-O-Ac-Iz and 5′-d(CCATC[Iz]CTACC products.

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